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“Smell” is an interesting verb. It can mean either the production or the reception of an olfactory signal. “The old dog smells bad” and “The old dog smells badly” are equally valid and meaningful sentences.

Birds smell, although not as consistently, in either sense, as dogs. Here’s a catalogue, from an article in The Auk by Julie Hagelin and Ian Jones: “[B]irds embrace a remarkable diversity of aromas, such as the musky plumage of storm-petrels; the tangerine-like perfume of Crested Auklets; the acrid, sour odor of Hooded Pitohuis and Variable Pitohuis; the sweet and dusty fragrance of the Kakapo; and the foul stench of the Hoatzin.”

Introductions may be in order here. Storm-petrels, the “Mother Carey’s chickens’ of sailors’ lore, are small pelagic birds related to albatrosses. Crested auklets are puffinlike North Pacific seabirds. Pitohuis, thrushlike songbirds native to New Guinea, are among the few known birds with toxic flesh. The kakapo is a large, flightless, nocturnal, nearly extinct New Zealand parrot. The hoatzin is a very odd, somewhat pheasantlike creature found in South American riverine forests; in Guyana it’s called “stinkbird.”

Later, Hagelin and Jones mention the “noxious scent” of the hoopoe. Other ornithologists have noted the characteristic musty smell of the Hawai’ian honeycreepers, reminiscent of an old canvas tent or, to some, earwax. The pioneer naturalist R.C.L. Perkins called it “disagreeable.”

These smells are various in origin. Some result from chemicals in the oil produced by the uropygial, or “preen,” gland at the base of the tail, which the bird spreads over its feathers. Hoatzins are victims of their digestive systems: they process leaves in an expanded foregut analogous to those of ruminant mammals. The reek of hoopoe droppings clings to both the birds and their nests, and likely repels potential predators.

It used to be conventional wisdom that birds had a poorly developed sense of smell, which would have been a mercy for hoopoes and hoatzins. A few exceptions, notably the turkey vulture, were acknowledged. Turkey vultures can detect carrion purely by scent and in fact have been recruited as pipeline inspectors, checking for leaks where the noxious gas mercaptan escapes. But Hagelin and Jones report that a fully functional olfactory system has been found in every bird species that has been studied. Birds, like mammals, live in and navigate a world of odors.

In addition to the turkey vulture, kiwis and tubenosed seabirds locate food by smell. The latter also use odor to identify their own nests in huge breeding colonies. Homing pigeons orient, at least in part, by smell. In a 1972 experiment, pigeons whose olfactory nerves had been snipped either never made it back to the loft or took longer than usual coming home. European blue tits avoid nest boxes that smell of weasels.

The possibility of avian pheromones—olfactory sex attractants—has generated much interest. So far the crested auklet remains the strongest candidate. Mature auklets of both sexes produce a tangerine aroma whose source has not yet been localized; the preen gland is not involved. In courtship, both partners perform what Hagelin and Jones call a “ruff sniff” display, placing their bills in their mates’ nape feathers. Auklets in the lab strongly oriented to tangerine-scented aldehydes, avoided mammalian musk, and were indifferent to the smell of bananas.

A team at the Chinese Academy of Sciences in Beijing recently reported that female budgerigars show a preference for the uropygial-gland odors of males over females. Apparently it takes a budgie to detect the difference. I had a pair for a while, and they both smelled the same to me.

The most recent lab findings, published in June in Behavioral Ecology, involve the familiar dark-eyed junco, the first songbird species to be investigated for preen-oil pheromones. Danielle Whittaker of Michigan State University worked with juncos from three geographically disjunct subspecies: “slate-colored” juncos from Virginia, “white-winged” juncos from the Black Hills of South Dakota, and “Oregon” juncos from the San Diego area. The three were once considered separate species and, given the vagaries of taxonomy, may well be again. Like the auklets and budgerigars, the juncos were subjected to maze tests that allowed them to demonstrate their olfactory preferences.

Those preferences, Whittaker et al wrote, were not in the predicted direction. Instead of preferring the scent of the opposite sex, both males and females preferred the scent of males. The scientists had also hypothesized that the juncos would prefer the smell of opposite-sex members of their own populations. But no; males failed to discriminate between Virginia, Dakota and San Diego females, while females favored exotic males. Finally, it was expected that females would opt for the scent of males with larger body size and/or more white on their tail feathers, both presumably badges of mate quality. Instead, females “spent significantly more time near the preen oil of males with smaller body size.”

So maybe females are prone to avoid the more macho-smelling males because they’re more aggressive? Maybe the male responses had as much to do with territorial defense as mate preference? Interesting complications.